1 Field of the Invention
The present invention relates to an electrically conductive composition, a ceramic electronic component, and a method for producing the component, and more particularly, to an electrically conductive composition for forming a conductive film on an outer surface of a ceramic electronic component through firing; a ceramic electronic component produced by use of the conductive composition; and a method for producing the component.
2. Description of the Related Art
Generally, an electrically conductive paste comprising solid components such as metal powder and glass frit and vehicle components such as a binder which provides appropriate viscosity and coatability and a solvent is employed as an electrically conductive composition so as to form an electrically conductive film on an outer surface of a ceramic electronic component, such as an outer metallized portion of a monolithic ceramic capacitor. Thus, an electrically conductive film is formed by provision of the electrically conductive paste on a target area of an outer surface of the body of an electronic component and firing.
In the case of a monolithic ceramic electronic component such as a monolithic ceramic capacitor, the body of the electronic component on which the electrically conductive film is formed includes an internal conductor formed therein and partially exposed so as to be able to establish electric contact with the electrically conductive film. Therefore, the electrically conductive paste is provided so as to make contact with the internal conductor and firing is performed to thereby establish electric contact between the internal conductor and metal contained in the conductive paste.
The use of glass frit in such an electrically conductive composition, e.g., a conductive paste, is herein divided into two cases. In a first case, there is employed a glass frit having a comparatively low softening point such that the frit assumes a molten state at least at the maximum temperature during firing. In a second case, there is employed glass frit having a comparatively high softening point such that the frit maintains a glass-ceramic state at the maximum temperature during firing.
In the first case in which there is employed glass frit having a comparatively low softening point, the glass component is softened at a comparatively early stage of the firing step. Therefore, even when an internal conductor is made of a base metal such as nickel, the metal contained in the electrically conductive composition and the internal conductor form an alloy, to thereby establish excellent electric contact between the conductive film made from the composition and the internal conductor, the base metal is oxidized.
When the maximum temperature during firing of the electrically conductive composition increases, the glass component exudes from the conductive film to the surface thereof. Therefore, when the conductive film is surface treatment plated, uniform adhesion of plating layer may be disturbed.
In contrast, when the maximum temperature during firing decreases, sintering of metal powder contained in the conductive composition proceeds insufficiently to thereby possibly provide a porous sintered product. Thus, during the plating treatment as described above, the plating solution might intrude into the ceramic body of the electronic component to thereby possibly cause degradation of electrical properties such as insulation resistance.
In the above-described second case in which there is employed a glass frit maintaining a glass-ceramic state at the maximum temperature during firing, the glass component does not melt during firing. Thus, good electric contact between an internal conductor and a metal contained in the electrically conductive composition might not be attained unless the internal conductor is sufficiently exposedxe2x80x94through a treatment such as polishing of the body of the electronic componentxe2x80x94before provision of the electrically conductive composition.
When an internal conductor contains a base metal, good electric contact between the internal conductor and a metal contained in the electrically conductive composition might be more difficult to attain since the internal conductor may be oxidized while the glass component contained in the conductive composition does not melt during the firing step.
To overcome the above described problems, the present invention provides an electrically conductive composition for forming an electrically conductive film through firing which comprises metal powder and glass frit as solid components, wherein the glass frit comprises first glass frit which assumes a molten state at least at the maximum temperature during firing and second glass frit which maintains a glass-ceramic state at the maximum temperature during firing.
One preferred embodiment of the present invention provides an electrically conductive composition for forming an electrically conductive film through firing comprising: metal powder and glass frit as solid components, wherein the glass frit comprises a first glass frit which assumes a molten state at least at the maximum temperature during firing and a second glass frit which maintains a glass-ceramic state at the maximum temperature during firing; the first frit has a softening point about 100xc2x0 C. or more lower than the maximum temperature during firing; the content of the first glass frit based on the total amount of the glass frit lies within a range of about 25-90 wt. %; and the total amount of the glass frit based on the total solid component including the metal powder lies within the range of about 3-10 wt. %.
In the above described electrically conductive composition, the metal powder is preferably at least one selected from the group consisting of copper and silver.
Another preferred embodiment of the present invention provides a ceramic electronic component comprising an electronic component body and an electrically conductive film disposed on an outer surface of the body through firing, wherein the electrically conductive film comprises a sintered product which contains metal and glass; the glass comprises a first glass which assumes a molten state at least at the maximum temperature during firing and a second glass which maintains a glass-ceramic state at the maximum temperature during firing; the first glass having a softening point about 100xc2x0 C. or more lower than the maximum temperature during firing; the content of the first glass based on the total amount of the glass lies within the range of about 25-90 wt. %; and the total amount of the glass in the conductive film lies within the range of about 3-10 wt. %.
In the above described ceramic electronic component, the metal powder is preferably at least one selected from the group consisting of copper and silver. Also, an internal conductor may be disposed in the electronic component body and electrically connected to the conductive film. Further, the internal conductor may comprise a base metal.
Yet another preferred embodiment of the present invention provides a method for producing a ceramic electronic component having an electronic component body and an electrically conductive film disposed on an outer surface of the body through firing, the method comprising the steps of providing the electronic component body; providing the electrically conductive composition as described above for forming a conductive film; disposing the electrically conductive composition on an outer surface of the electronic component body; and firing the conductive composition such that the maximum temperature during firing is about 100xc2x0 C. or more higher than the softening point of the first glass frit and the second glass frit maintains a glass-ceramic state at the maximum temperature during firing.
In the above described method, an internal conductor may be disposed in the electronic component body and partially exposed so as to be electrically connected to the electrically conductive film; and the step of disposing the electrically conductive composition includes a step of providing the electrically conductive composition so that the electrically conductive composition contacts the internal conductor. Further, the internal conductor may comprise a base metal.